As Martin says, 'How fast do you want to go'?
A 998 Mini will cruise at 70+ mph all day long. Even the original 850 would cruise at over 60 mph.
If you want to cruise at 90+ mph for long distances on big main roads a classic Mini, or indeed most classic cars, will leave you disappointed. A 58-year-old design isn't going to be fast, unless you make it so radical it is no longer a classic Mini in spirit or in fact.
The real joy with the classic Mini is the 1960's retro driving experience. By all means give it Cooper 'S' Mk.1 performance with a nice 1275 engine, but if you go too far with the modifications it can become (even more) unreliable and harsh.
Do the suspension & brakes, then slightly improve the engine to give better mid-range torque and it will be a simply delightful car to drive & to own.